(a) Field of the Invention
The present invention relates to the average bubble correction circuit in the analog to digital converter. The average bubble correction circuit can convert the thermometer code obtained from the comparator of the analog to digital converter into the 1/0 state-conversion point.
(b) Description of the Prior Arts
In the deep submicron technology, the channel length will be shorter and the breakdown voltage will be decreased when the device is made smaller. Therefore, in order to improve the reliability of the circuit and to obtain the better yield of the product, the working voltage has to be decreased to a certain value. The device whose channel length (xcex) is 3.5 micrometers can take the working voltage 3.3 V. However, when the channel length (xcex) of the device is decreased to 2.5 micrometers, the working voltage 3.3 V will cause the problem of reliability. The working voltage has to be decreased to 2.5 V accordingly to maintain the reliability when the channel length is changed to 2.5 micrometers. In general, the working voltage needs to be decreased as the manufacturing process advanced. It is the nature in physics that the decrease of the working voltage is proportional to the decrease of the channel length. Example that when the channel length (xcex) decreasing from 3.5 micrometers to 2.5 micrometers, the working voltage has to be decreased from 3.3 V to 2.5 V. Another factor is the threshold voltage that affects the working voltage, the threshold voltage is not changing in proportion with the change of the channel length (xcex) accordingly. For example, when the channel length (xcex) changing from 3.5 micrometers to 2.5 micrometers (the ratio is 0.09), the threshold voltage will change from 0.7 V to 0.56 V (the ratio is 0.8), which means, for a designer of a circuit, the linear voltage range is decreased, for example, from 1.5V to 1 V.
The main disadvantages of the analog to digital converter are as follows: (1) signal range (2) resolution (N bits) (3) random offset voltage of the device. The least significant bit voltage VLSB can be represented as VLSB=Vxcex7/2N when the signal range is Vxcex7 and the resolution is N bits.
The practical circuit has to resolute a least significant bit voltage VLSB (strictly, to xc2xdVLSB) in order to meet the demand of the N-bit analog to digital converter. From the formula above, the least significant bit voltage VLSB is proportional to the signal range Vxcex7 and the signal range Vxcex7 has to be in the linear voltage range. In advanced manufacturing process, the signal range Vxcex7 and the least significant bit voltage VLSB will be decreased when the working voltage decreased.
The random offset voltage Vos of MOS and the thermal noise Vn are the bottlenecks of decreasing the least significant bit voltage VLSB. Any circuit will not detect the degree of the signal when the least significant bit voltage is smaller than the thermal noise or is smaller than the random offset voltage. With the auto correction circuit or the offset calibration circuit, the better resolution can be obtained. Example that the signal range is 1.5 V when the working voltage is 3.3 V in the process whose channel length is 3.5 micrometers, the least significant bit voltage of the 8-bit analog to digital converter is equal to 5.86 mV and the random offset voltage is between 10 mV and 20 mV, obviously, the VLSB is much smaller then the Vos. Therefore, the probability of the generating of the bubble will increase for the flash analog to digital converter. And, vary likely; several bubbles will be generated in the same time.
The effects of the bubbles are as follows: (1) the error of the decoding of read only memory (ROM) (2) the decreasing of the rate of the noise of the signal (3) the increasing of the bit error rate. When the bubbles are more than one, the number of the inputs of the decoder which is preset will be more than one xe2x80x9c1xe2x80x9d and will not be corrected, and further result in the error of the binary code of the outputs. And the current passing through the transistor will increase extremely and the bit error rate will cause the data sampling fail to meet the specification. The yield will not be improved and the cost will increase. So, the better error correction circuit is vary important for the analog to digital converter in the advanced manufacturing process.
FIG. 1 is showing the schematics of the flash analog to digital converter in prior art. The output of the comparator 12 in the flash analog to digital converter 10 is the thermometer code. If the input voltage of the comparator is higher than the reference voltage, the logical output is xe2x80x9c1xe2x80x9d. If lower, the logical output will be xe2x80x9c0xe2x80x9d. The analog signal converted to the digital signal is shown in FIG. 2. The analog signal outputted from a series of the comparators 12 will be similar to the thermometer code style in FIG. 2. In order to form the thermometer code style, all xe2x80x9c0xe2x80x9d will be above the input voltage and all xe2x80x9c1xe2x80x9d will be under the input voltage. The boundary of the xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d can reflect the degree of the input voltage. The thermometer code will be converted to 1-OF-N code by 1/0 state-conversion detector and will be decoded to binary code by ROM 14.
Because of (1) high slew rate input (2) the difference of the clock pulse distribution (3) process offset, the micro difference of the random input offset voltage and the response time of the comparators will result in one or many xe2x80x9c0xe2x80x9ds appear in a series of xe2x80x9c1xe2x80x9d, or one or many xe2x80x9c1xe2x80x9ds appear in a series of xe2x80x9c0xe2x80x9d, and this is so called bubble error.
When the errors happened in the analog to digital converter, two or more 1/0 state-conversion points will appear, and which will cause errors occur in the process of decoding and further cause the increasing of the bit error rate and the decreasing of the noise signal rate of the analog to digital converter. The problem of the error code of the bubble can be solved by using three-end input logical device that referring three continuous thermometer codes.
FIG. 3A is showing the diagram of the three-end input logical device which can detect the 1/0 state-conversion point with input xe2x80x9c001xe2x80x9d. When the thermometer codes that input to the AND gate 30 of the three-end logical input device is xe2x80x9c001xe2x80x9d and the output signal of the AND gate 30 is xe2x80x9c1xe2x80x9d, it is indicating that the 1/0 state-conversion point has been detected, otherwise, the output signal of the AND gate 30 is xe2x80x9c0xe2x80x9d.
FIG. 3B is showing the diagram of the three-end input logical device which can detect the 1/0 state-conversion point with input xe2x80x9c011xe2x80x9d. When the thermometer codes that input to the AND gate 32 of the three-end logical input device is xe2x80x9c011xe2x80x9d and the output signal of the AND gate 32 is xe2x80x9c1xe2x80x9d, it is indicating that the 1/0 state-conversion point has been detected, otherwise, the output signal of the AND gate 30 is xe2x80x9c0xe2x80x9d.
The thermometer codes will be converted to 1-OF-N codes by using the three-end logical input device and will be further decoded into the binary codes.
FIG. 4 is showing the condition that the bubble being detected by the three-end logical input device in prior art. The disadvantage of the three-end logical input device is that the 1/0 state-conversion point can be detected, but the result is not the best one. For example, in FIG. 4, the three-end logical input device will use xe2x80x9c011xe2x80x9d or xe2x80x9c001xe2x80x9d to detect the 1/0 state-conversion point, and the result is showing in the (a) series and (b) series thermometer codes in FIG. 4. When the thermometer codes of the comparator 40 and the comparator 42 generate the bubbles, the three-end logical input device will not detect which comparator generating the errors. As shown in FIG. 4, the 1/0 state-conversion points that detected by the (a) series and the (b) series thermometer codes are not the proper ones, however, the (c) series thermometer codes will generate the best result.
FIG. 5B and FIG. 5C are showing the several kinds of results that the three-end logical input device that detecting bubbles in prior art. FIG. 5A is showing the six combinations of the thermometer codes. The result of the test of the three-end logical input device in the condition of xe2x80x9c001xe2x80x9d is shown in FIG. 5B. The result of the test of the three-end logical input device in the condition of xe2x80x9c011xe2x80x9d is shown in FIG. 5C. As shown, when the bubbles are over one, the three-end logical input device will not correct the error properly. More, when the number of the bubble is 1, but the depth of the number is over 2, the three-end logical input device only can correct the error in one direction. As shown in FIGS. 5B and 5C, with the condition xe2x80x9c001xe2x80x9d, the three-end logical input device can correct the thermometer codes of the (a) series, the (b) series and the (d) series, but can not correct the thermometer codes of the (c) series, the (e) series and the (f) series since the depth of the bubbles in the (c) series and the (e) series are over 2 and the (f) series generating two bubbles. With the condition xe2x80x9c011xe2x80x9d, the three-end logical input device can correct the thermometer codes of the (a) series, the (c) series and the (e) series, but can not correct the thermometer codes of the (b) series, the (d) series and the (f) series since the depth of the bubbles in the (b) series and the (d) series are over 2 and the (f) series generating two bubbles. As seen, the three-end logical input device only can correct in one direction (up or down) with depth 2 of the error and with only one bubble.
The primary aspect of the present invention is to provide an average bubble correction circuit which will expand the range of bubble error correction and will detect the proper position of the 1/0 state-conversion points of the thermometer codes to low down the error rate that caused by the ROM decoding.
In order to achieve the goals described above, the present invention relates to an average bubble correction circuit comprising a plurality of bubble correction cells that receiving the thermometer codes output from several comparators in an analog to digital converter. The bubble correction cells will detect the output of the thermometer codes and output a set of debugged codes to a state detecting circuit; which can be composed of several conventional Exclusive-Or gate or the three-end logical input device described above, to obtain a set of 1-OF-N codes. The set of 1-OF-N codes will be further input into a decoder and converted to a binary code. Any of the bubble correction cells will select adjacent 2m+1 (m is a natural number) thermometer codes from the plurality of thermometer codes, and determine the state that coming out most often and output a code indicating the state that coming out most often.
Another aspect of the present invention is to provide a bubble correction circuit. With the selection model, the present invention will regard the rare-happened signal xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d as bubble and to determine the proper position of the 1/0 state-conversion points of the thermometer codes low down the error rate that caused by the ROM decoding. The present invention also expands the range of bubble error correction to cope with the problem of increasing bubble that happened in micro manufacturing process.
The appended drawings will provide further illustration of the present invention, together with the description, serve to explain the principles of the invention.